The present invention relates generally to automatic transmissions constructed to be shifted through a gear shift accompanied by a shift of a first friction engagement element from an engaged state into a disengaged state and a shift of a second friction engagement element from a disengaged state into an engaged state, and relates particularly to automatic transmission control apparatuses and methods for controlling a torque capacity of the first or second friction engagement element during an inertia phase of the gear shift.
Japanese Patent Application Publication No. 2002-089691 (henceforth referred to as JP2002-089691) corresponding to U.S. Pat. No. 6,616,576 discloses an automatic transmission control apparatus for an automatic transmission. During an inertia phase of a downshift or upshift of the automatic transmission, the automatic transmission control apparatus calculates a desired torque capacity of a friction engagement element to be engaged or disengaged for the downshift or upshift, on a basis of an actual transmission gear ratio that is calculated on a basis of an input speed and an output speed of the automatic transmission, and controls the friction engagement element by outputting a hydraulic pressure command so as to achieve the desired torque capacity.
Inertia phase or inertia control phase is a phase of a gear shift of an automatic transmission in which an inertial force of a gear train of the automatic transmission plays a major role in changing an input speed of the automatic transmission.
The calculation of the desired torque capacity is implemented by interpolation from values of the desired torque capacity corresponding to a plurality of reference transmission gear ratios. For example, the reference transmission gear ratios include a transmission gear ratio at a starting point of the inertia phase, a transmission gear ratio at an end point of the inertia phase, and a transmission gear ratio at an intermediate point of the inertia phase. Such calculation of a desired torque capacity of a friction engagement element is henceforth referred to as transmission gear ratio interpolation calculation or transmission gear ratio interpolation operation. The transmission gear ratio interpolation operation is performed during the inertia phase from a time instant when the actual transmission gear ratio starts to change in a direction conforming to the downshift or upshift to a time instant when the change of the actual transmission gear ratio in the direction is completed.